Siloxane polymers have unique properties derived mainly from the physical and chemical characteristics of the siloxane bond. These properties include low glass transition temperature, thermal and oxidative stability, resistance to ultraviolet radiation, low surface energy and hydrophobicity, high permeability to many gases, and biocompatibility. The siloxane polymers, however, often lack tensile strength.
The low tensile strength of the siloxane polymers can be improved by forming block copolymers. Some block copolymers contain a “soft” siloxane polymeric block or segment and any of a variety of “hard” blocks or segments. Polydiorganosiloxane polyamides, polydiorganosiloxane polyureas, and polydiorganosiloxane polyoxamide copolymers are exemplary block copolymers.
Polydiorganosiloxane polyamides have been prepared by condensation reactions of amino terminated silicones with short-chained dicarboxylic acids. Alternatively, these copolymers have been prepared by condensation reactions of carboxy terminated silicones with short-chained diamines. Because polydiorganosiloxanes (e.g., polydimethylsiloxanes) and polyamides often have significantly different solubility parameters, it can be difficult to find reaction conditions for production of siloxane-based polyamides that result in high degrees of polymerization, particularly with larger homologs of the polyorganosiloxane segments. Many of the known siloxane-based polyamide copolymers contain relatively short segments of the polydiorganosiloxane (e.g., polydimethylsiloxane) such as segments having no greater than 30 diorganosiloxy (e.g., dimethylsiloxy) units or the amount of the polydiorganosiloxane segment in the copolymer is relatively low. That is, the fraction (i.e., amount based on weight) of polydiorganosiloxane (e.g., polydimethylsiloxane) soft segments in the resulting copolymers tends to be low.
Polydiorganosiloxane polyureas are another type of block copolymer. Although these block copolymers have many desirable characteristics, some of them tend to degrade when subjected to elevated temperatures such as 250° C. or higher.
Polydiorganosiloxane polyoxamides such as those disclosed in U.S. Pat. No. 7,501,184 (Leir et al.) are yet another type of block copolymer. Known polydiorganosiloxane polyoxamide copolymers have been made by mixing a diamine such as ethylene diamine with a precursor that includes at least one polydiorganosiloxane segment and at least two oxalylamino groups. The resulting copolymers have alternating soft polydiorganosiloxane segments (S) and hard oxamide segments (H) (i.e., the copolymers are of a (S—H)n type). These polydiorganosiloxane polyoxamide copolymers thus contain a relatively large fraction of the polydiorganosiloxane segment compared to many known polydiorganosiloxane polyamide copolymers. Such polydiorganosiloxane polyoxamide copolymers can usually be subjected to elevated temperatures up to 250° C. or higher without apparent degradation.